CdS/Low-Band-Gap Kesterite Thin-Film Solar Cell Absorber Heterojunction: Energy Level Alignment and Dominant Recombination Process

The chemical and electronic interface structure (including energy level alignment) between wet-chemical deposited CdS and low-band-gap Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cell absorbers was studied using direct and inverse photoemission. Complementarily, the activation energy (Ea) of the dominant charge carrier recombination process of related solar cell devices was derived by temperature-dependent current−voltage [I(V,T)] measurements. We find the CZTSSe surface to be free of any significant amount of sulfur and the formation of Cd−Se bonds at the interface. A small, positive (“spike”-like) conduction band offset of 0.21 ± 0.28 eV between CZTSSe and CdS was measured. In conjunction with the I(V,T) derived Ea of 1.09 ± 0.07 eV, which is in excellent agreement with the CZTSSe bulk band-gap energy of 1.07 eV, this reveals that high-rate charge carrier recombination at the CdS/CZTSSe interface can mainly be excluded as the performance-limiting factor in corresponding solar cells.